Raissa F. P. Mendes

Possibility of setting a new constraint to scalar-tensor theories

Scalar-tensor theories (STTs) are a widely studied alternative to general relativity (GR) in which gravity is endowed with an additional scalar degree of freedom. Although severely constrained by solar system and pulsar timing experiments, there remains a large set of STTs which are consistent with all present day observations. In this paper, we investigate the possibility of probing a yet unconstrained region of the parameter space of STTs based on the fact that stability properties of highly compact neutron stars in these theories may radically differ from those in GR.

Instability of nonminimally coupled scalar fields in the spacetime of slowly rotating compact objects

containing unstable modes in a background which is flat in the asymptotic past and stationary and axially symmetric in the future. In Sec. III we present a simple general argument that shows that the parameter space which characterizes the instability is not modified at first order in the compact object’s angular momentum. Then, we investigate secondorder deviations from staticity in a particular model, taking as the source of the gravitational field a class of slowly spinning shells. The general properties of the shell spacetime are presented in Sec. IV. Considering spinning thin shells allows us to push the analytical treatment further and arrive at clear conclusions about the role played by rotation on the instability. This is pursued in Sec. V. Section VI is devoted to a discussion of the results and to our final remarks. We assume metric signature ð− þ þþÞ and natural units in which c ¼ G ¼ ℏ ¼ 1 unless stated otherwise.

Highly compact neutron stars in scalar-tensor theories of gravity: Spontaneous scalarization versus gravitational collapse

Scalar-tensor theories of gravity are extensions of General Relativity (GR) including an extra, nonminimally coupled scalar degree of freedom. A wide class of these theories, albeit indistinguishable from GR in the weak field regime, predicts a radically different phenomenology for neutron stars, due to a nonperturbative, strong-field effect referred to as spontaneous scalarization. This effect is known to occur in theories where the effective linear coupling

Awaking the vacuum with spheroidal shells

\beta_0

Tidal deformability of boson stars and dark matter clumps

between the scalar and matter fields is sufficiently negative, i.e.

Instability of nonminimally coupled scalar fields in the spacetime of thin charged shells

\beta_0 \lesssim -4.35

Self-force and fluid resonances

, and has been strongly constrained by pulsar timing observations. In the test-field approximation, spontaneous scalarization manifests itself as a tachyonic-like instability. Recently, it was argued that, in theories where

Radiation interference from sources rotating around Schwarzschild black holes

\beta_0>0

Reply to "Comment on `Quantum versus classical instability of scalar fields in curved backgrounds'"

, a similar instability would be triggered by sufficiently compact neutron stars obeying realistic equations of state. In this work we investigate the endstate of this instability for some representative coupling functions with

New Class of Quasinormal Modes of Neutron Stars in Scalar-Tensor Gravity

\beta_0>0